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Grid emulators provide efficient manners to test grid-connected distributed power generators. They test the power generator such as photovoltaics, wind turbines without hardware damages. Recent studies investigated conventional voltage source inverters such as a two-level voltage source inverter and a three-level voltage source inverter as grid emulators. However, they have limited voltage capabilities and weak transient responses. This paper presents a technique for the control of a modular multilevel converter as a grid emulator in fault scenarios, where voltage sags, harmonics distortions, and frequency deviations are emulated. The steady-state performance in these scenarios is analyzed and compared with that of a two-level voltage source inverter. Energy balancing between the sub-modules in a modular multilevel converter is always a crucial topic in terms of stability, particularly under fault conditions. In this paper, the transient response of the point of common coupling voltage and the sub-module energy in the modular multilevel converter with a delta-wye configuration is analyzed and compared with a two-level voltage source inverter. Simulations of a 24 sub-modules modular multilevel converter verify the control method with S70 V at the dc-link and a 400 V line-to-line ac voltage at the point of common coupling. The modular multilevel converter grid emulator with a delta-wye transformer configuration shows better steady-state performances than a two-level voltage source inverter. The energy stored in each sub-module is well balanced during the fault scenarios, which indicates strong stability.

Power transformers using a high temperature superconductor (HTS) ReBCO coated conductor and liquid nitrogen dielectric have many potential advantages over conventional transformers. The ac loss in the windings complicates the cryogenics and reduces the efficiency, and hence it needs to be predicted in its design, usually by numerical calculations. This article presents detailed modelling of superconducting transformers with Roebel cable in the low-voltage (LV) winding and a high-voltage (HV) winding with more than 1000 turns. First, we model a 1 MVA 11 kV/415 V 3-phase transformer. The Roebel cable solenoid forming the LV winding is also analyzed as a stand-alone coil. Agreement between calculations and experiments of the 1 MVA transformer supports the model validity for a larger tentative 40 MVA 110 kV/11 kV 3-phase transformer design. We found that the ac loss in each winding is much lower when it is inserted in the transformer than as a stand-alone coil. The ac loss in the 1 and 40 MVA transformers is dominated by the LV and HV windings, respectively. Finally, the ratio of total loss over rated power of the 40 MVA transformer is reduced below 40% of that of the 1 MVA transformer. In conclusion, the modelling tool in this work can reliably predict the ac loss in real power applications. This is the Accepted Manuscript version of an article accepted for publication in 'Superconductor Science and Technology'. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/0953-2048/28/11/114008.

The commercial application of conventional unified power flow controllers (UPFCs) is rare, because of their high installation and operational costs. Sen proposed the “Sen” transformer (ST) that provides similar power flow control functionality as UPFCs, using only a multiwinding transformer and tap changers. However, the ST has output error and response speed issues, due to the discrete characteristics of tap changers. A novel hybrid electromagnetic unified power flow controller (HEUPFC) is proposed. The HEUPFC is composed of a larger capacity ST and a smaller capacity UPFC. The HEUPFC provides economy, flexibility, high reliability, and continuously variable amplitude and phase angle output. The minimum ratio of the magnitude of the series injected voltage from the UPFC is deduced for several center tap cases. The proposed HEUPFC can control the bulk ST power flow with coarse step changes and precise UPFC control, and precisely control active and reactive power flow. Simulation and experimental results were used to verify the practicability of the HEUPFC.

This paper presents a high-frequency modular medium-voltage AC (4160 VAC or 13.8 kVAC) to low-voltage DC (400 VDC) system that is scalable in order to be used for different scale microgrids. A 15 kW, 500 kHz DC/DC converter is demonstrated as the most important stage of the system overall, which can be scalable to a 225 kW 4160 VAC to 400 VDC system. Motivation of a CLLC resonant converter and its design parameters determination is carefully illustrated. The high-frequency transformer is the key element for the DC/DC converter. Then, the paper focuses on high-frequency transformer design to realize high-voltage insulation, high efficiency, and high density at the same time. Based on a split winding UU core transformer structure, transformer insulation materials and dimension parameters are determined referring to the IEEE insulation standard. The transformer magnetic loss model is reviewed based on which loss design tradeoff is carefully analyzed. Different working frequency impact on transformer design over three different core materials is also presented. Finally, a 500 kHz transformer prototype has been developed and demonstrated with the IEEE standard required insulation tests. A whole CLLC resonant converter is also present with experiments results. The converter holds outstanding 98% efficiency and 2.9 kW/L power density.

A new method is implemented in designing of self-oscillating loop for driving piezoelectric transformers. The implemented method is based on combining both analog and digital control systems. Digitally controlled time delay through the self-oscillating loop results in very precise frequency control and ensures optimum operation of the piezoelectric transformer in terms of gain and efficiency. Time delay is implemented digitally for the first time through a 16 bit digital-to-analog converter in the self-oscillating loop. The new design of the delay circuit provides 45 ps time resolution, enabling fine-grained control of phase in the self-oscillating loop. This allows the control loop to dynamically follow frequency changes of the transformer in each resonant cycle. Ultimately, by selecting the optimum phase shift, maximum efficiency under the load and temperature condition is achievable.

A novel indirect measurement method to estimate the equivalent air-core inductance of an iron-cored transformer winding is proposed. The method utilizes frequency response analysis (FRA) magnitude data to estimate this quantity. Its implementation involves just two FRA measurements: one with the neutral end open circuited, whereas the other one is with a known external lumped capacitance connected between the neutral and ground. This equivalent inductance is straightforward to estimate from the short-circuit natural frequencies extracted from the two FRA magnitude plots. The recently derived analytical relationship connecting short-circuit natural frequencies to winding inductances and capacitances is employed for this purpose. Experimental demonstration is reported initially for actual windings with core removed to prove the feasibility of the method, and later, for an actual transformer. Finally, experiments on an air-cored winding are also reported to highlight one possible use of this quantity in discriminating radial and axial displacements.

The soft-switching solid-state transformer (S4T) employs only 12 main active devices and an auxiliary resonant circuit to implement a bidirectional solid-state transformer, with an attractive feature of achieving a full range of zero-voltage-switching conditions for all the main devices. This paper covers detailed design of the power stage, auxiliary resonant circuit, and control of the S4T. The high-frequency transformer is an essential element for the S4T, and it has a unique feature of dc-biased flux. Design of such a high-frequency transformer is also discussed in detail in this paper. Soft startup, shutdown, and fast dynamic response under load transients are also attractive behaviors because of the low inertia of the S4T. Experimental results from a 208-V/10-kV·A S4T unit are presented.

A self-calibrating current transformer with nominal ratio 10 A to 100 mA, intended for low frequencies applications, was developed. It has 100 independent primary windings to be connected in series or parallel, so that it can be configured as 100:1 or 1:1 transformer (all windings in series). This last ratio permits autocalibration. Error sources are analyzed, and it is shown that differences between parallel and series configurations have uncertainties lower than 1 part in 106.

It is well known that the most polluted networks with harmonics are those of low voltage. As the voltage level increases toward the high voltage power supply, the current and consequently voltage harmonics level naturally decrease. The goal of the paper is the study of the parameters’ influence (of power lines and transformers) and also of the type of connection of power transformers on the harmonics propagation in upstream from a low voltage level to higher level when no special mitigation measures are taken. A sensitivity study was also performed regarding the accuracy of transformers’ modeling with respect to the obtained results.

The objective of this paper is to discuss about the requirement of Negative Phase Sequence (NPS) overcurrent protection for solidly grounded Delta-Wye transformer and to provide guidelines to select the setting for NPS overcurrent protection. Currently NPS overcurrent protection is not used to protect the static device like transformers in any industrial projects. However for certain risk related to failure of protection equipment, only NPS overcurrent protection can provide backup protection to solidly grounded Delta-Wye transformers. Hence NPS overcurrent protection function can't be avoided in transformer protection relay at delta side of transformer. The comparison between all the transformer protections based on failure of certain protection equipment under fault condition is also discussed in this paper.

Precise localization of partial discharge (PD) inside a power transformer winding is a challenging task. Previously, researchers have used internal calibration or reference signals to locate the PD source. The location of any arbitrary (test) PD source is ascertained from the maximum correlation between reference and test signals. However, in practical transformer windings, internal tappings or design details are usually unavailable to generate reference signals. The proposed work employs terminal measurements to construct a physically realizable ladder network. Simulated responses obtained from the ladder network for signals of known pulse-widths at all locations are used as reference data. The terminal responses of the test PD signals are obtained from a laboratory-scale winding by applying signals of arbitrary pulse-widths and shapes at various locations. The PD test signals are generated using a function generator, a PD calibrator, and real discharges. To predict the location of the PD source, the simulated reference data are then correlated with the test data. The position corresponding to the maximum correlation indicates the PD location. The proposed methodology is verified using experimental investigations carried out on two different laboratory-scale transformer windings.

Current transformer (CT) saturation can cause the maloperation of transformer differential protection under external faults. To avoid this problem, a method based on the histogram of the normalized rotated current (NRC) is proposed. NRC is obtained by a symmetric waveform transformation of a part differential current. An index based on the histogram is defined to evaluate the NRC frequency distribution. The external faults with different degrees of CT saturation, internal faults, and transferring faults can be clearly distinguished by the proposed method. Moreover, the detection time is invariably less than a half cycle after the obvious increase of the differential current. Simulation results validated the proposed method.

This paper presents a new topology for a fully bidirectional soft-switching solid-state transformer (S4T). The minimal topology, featuring 12 main devices and a high-frequency transformer, does not use an intermediate dc voltage link, and provides sinusoidal input and output voltages. The S4T can be configured to interface with two- or multiterminal dc, single- or multiphase ac systems. An auxiliary resonant circuit creates zero-voltage-switching conditions for main devices from no-load to full-load, and helps manage interactions with circuit parasitic elements. The modularized structure allows series and/or parallel stacking of converter cells for high-voltage and high-power applications.

This paper proposes a three-phase lines to single-phase coil planar contactless power transformer. The transformer has the primary consisting of three-phase line cables arrayed in parallel on a plane and the secondary of single-phase coil. The proposed transformer is studied for application to automatic guided vehicles (AGVs). Uniform power transfer that is independent of the position of the AGVs is an important issue for this transformer, because of the sparse arrangement of the primary lines. To ensure that the transformer achieves uniform power transfer, the structure and dimensions of the primary are studied using numerical analysis. Additionally, an equivalent model of the proposed transformer, which is applicable to the imbalanced conditions, is presented. The equivalent model proves analytically that balanced three-phase voltage and balanced three-phase current conditions, which can be created using appropriate circuit configurations, enable uniform power transfer. The results of experiments in which the prototype of the transformer is applied to the target AGV confirm that under balanced-current conditions, which can be generated using a three-phase LCL circuit, the proposed transformer can transfer sufficient uniform power to drive the AGV.

Power transformer, as one of the most important apparatus in power system, must be protected against the turn-to-turn faults. Although the various methods based on the terminal currents and/or voltages have been presented for transformer protection, the flux-based methods can be used to achieve the more accurate, sensitive, and secure results. Since the symmetrical form of the magnetic flux distribution will be disturbed due to any fault occurrence in the transformer windings, it can be considered as an appropriate criterion to achieve a suitable protection algorithm, which can detect the fault occurrence and identify the faulty phase/region. In this paper, a new leakage flux sensor is introduced and then a flux-based technique is proposed to detect and identify the faulty phase in transformers to apply in online conditions. By using the proposed technique, the faulty region can be identified in offline conditions, as well. The experimental results (which are done on a distribution transformer) show that this technique is secure in the face of overfluxing and transformer energizing conditions and can detect turn-to-turn faults with high sensitivity. Moreover, tap changer operation, imbalanced loads, and/or unsymmetrical voltage sources cannot influence the proposed technique security.

This paper proposes a bidirectional isolated single-phase matrix converter with a center-tapped transformer, which is used in a battery storage system connecting to a single-phase ac load. The proposed converter consists of a full-bridge inverter, a high-frequency center-tapped transformer, and a matrix converter. The matrix converter is employed with a pulse density modulation in order to achieve zero-voltage switching (ZVS). On the other hand, a common-mode voltage of the center-tapped transformer and a small capacitor are utilized in order to absorb the power ripple of the single-phase ac load without additional switches. Furthermore, the switches in the full-bridge inverter partially achieve ZVS in order to reduce the switching loss. These characteristics result in a high-power-density converter. As the experimental results with a 1 kW-prototype, the proposed converter reduces the dc current ripple generated by a single-phase ac load by 80.2% in the discharging mode and 76.8% in the charge mode. Besides, the low ac-side voltage total harmonic distortion in the discharge are below 3.0% over the entire ac power range.

In this paper, we propose an approach for integrating a series filter inductance in a coaxially wound transformer (CWT) as a potential alternative to conventional solenoidally wound transformers (SWT) for high-power isolated soft-switching dc–dc converters. The critical elements that determine the size and the performance of soft-switching dc–dc converters are the isolation transformer and the filter elements. While conventional SWTs using a stray magnetic field as a filter inductance often suffer from a considerably increased loss, heat congestion, and electromagnetic interference, a CWT is rarely affected by high frequency and fringing effects. The desired properties are still valid while the required filter inductance is fully integrated with an isolation transformer using the proposed method. In addition, the evenly distributed magnetic fields in a concentric geometry enable accurate electrical and mechanical analysis and a specific application-oriented design. In this paper, the operating principles, design procedure, and loss modeling of a CWT with an integrated filter inductance are introduced. Its functionality and suitability for high-frequency applications were proven by theoretical analysis and experiments. A prototype transformer with an integrated inductance 4 mH was facilitated and evaluated in a 15 kV-SiC mosfet -based 6 k–400 Vdc stage for a solid-state transformer of up to 6.5 kW at a switching frequency of 20 kHz.

This paper describes an improved transformer winding tap injection distribution static synchronous compensator (WTI-DSTATCOM) for medium-voltage reactive power compensation. The cascaded multilevel converter based DSTATCOM is connected to the special-designed winding taps on the primary windings of the transformer instead of the conventional point of common coupling. The voltage stress for DSTATCOM to handle is reduced. The winding tap injection (WTI) method can make full use of the spare capacity of the transformer and obtain a flexible connection voltage for DSTATCOM. The compensation mechanism and winding current distribution after currents injection are analyzed by phasor diagrams under steady state. A nonlinear passivity-based control (PBC) algorithm is designed for inner-loop current control and a three-layer voltage balancing control strategy is applied to balance the dc capacitor voltage. The results obtained from the MATLAB/Simulink simulations and a down-scaled laboratory prototype experiment of 800 V verify the feasibility and effectiveness of the proposed WTI-DSTATCOM system with PBC algorithm in reactive power compensation.

The transformer-energization inrush phenomenon is to be expected in cases when a single transformer is connected to an ac voltage source. The most commonly employed mitigation technique is to use mechanisms that enable per-phase controlled switching based on a residual magnetic-flux estimation in the transformer’s ferromagnetic iron core. However, the time scale of the phenomenon is relatively short and imposes no extremely critical issues to wide-scale power-system operation. On the other hand, the sympathetic inrush phenomenon that might occur among several parallel transformers exhibits much longer time constants (several seconds and even tens of seconds). The resulting system impact becomes longer lasting. Recently, national wide area monitoring system (WAMS) measurements indicated that a sympathetic interaction occurred between two parallel 400/110-kV transformers in the Slovenian power system. The cause was the magnetization of a second transformer in parallel to the already operational and fully loaded one in the substation in electrical vicinity to a large power plant. Apart from the WAMS, the event was also captured by the transformer’s over-current protection relay. The specifics of the event are described in this paper together with a dynamic-simulation-based reconstruction that is very important for the operator’s understanding of on-line, WAMS-provided, and system measurements.

This letter presents a 20-pulse multiphase staggering autoconfigured transformer (MSAT) for power quality improvement in medium and high power applications. The proposed autoconfigured transformer is a combination of delta and zig-zag transformers. Contrary to the prior art multipulse autoconnected transformers, magnetic rating of the proposed configuration is only 40% of the load rating in addition to improved power quality indices. Moreover, the creation of a neutral point ascribed to inbuilt zig-zag windings supplements as one of the remarkable advantages of the proposed MSAT. Therefore, with this suggested configuration, the performance of zig-zag transformer is accomplished within the autoconfigured transformer. A detailed comparison of the proposed MSAT configuration with that of other multipulse autoconnected transformers in terms of magnetic rating and power quality indices are presented. Further, the feasibility and operability of the proposed MSAT are verified and validated through experimental tests on the laboratory prototype. The test results so obtained complies with the IEEE-519 and IEC 61000-3-2 standards on power quality requirements.

Classical DC current transformer, based on a linear amplifier as a compensating amplifier, is well known as a power inefficient device. If the linear amplifier is replaced with switching counterpart, significant power saving can be achieved. A natural choice for replacement can be found in a half-bridge class-D amplifier. However, the half-bridge class-D amplifier is prone to bus-pumping effect, and direct replacement cannot be performed in a straightforward manner. This study presents an analysis of power consumption issues and irregular behaviour for a DC current transformer, caused by the bus-pumping effect. For the case of DC current measurement, the main issues are explained, giving better insight into power consumption, and possible hazards to transformer circuitry. Presented analytical results are experimentally verified.

In this paper, an input-series two-transistor forward converter is proposed and investigated, which is aimed at high-input voltage multiple-output applications. In this converter, all of the switches are operating synchronously, and the input voltage sharing (IVS) of each series-module is achieved automatically by the coupling of primary windings of the common forward integrated transformer. The active IVS processes are analyzed based on the model of the forward integrated transformer. Through the influence analysis when the mismatches in various series-modules are considered, design principles of the key parameters in each series-module are discussed to suppress the input voltage difference. Finally, a 96-W laboratory-made prototype composed of two forward series-modules is built, and the feasibility of the proposed method and the theoretical analysis are verified by the experimental results.

This paper presents a study of lightning overvoltages transferred to the low-voltage side of a 10 kVA 7.967 kV/240–120 V single-phase distribution transformer considering dispersive ground effects in the medium-voltage distribution line terminated at its primary side. A two-port wideband model that is capable to represent the transformer behavior under different load conditions is considered. The medium-voltage distribution line is modeled using Marti’s transmission line model modified to include the variation with frequency of the ground resistivity and permittivity. Periodical grounding points are also modeled considering dispersive ground effects. Phase-neutral voltages transferred to the transformer secondary are calculated for direct lightning strikes at the medium-voltage line including insulation breakdown, surge arresters, and different load conditions. It is shown that the frequency variation of the ground resistivity and permittivity can significantly affect voltages transferred to the low-voltage side of the transformer if a high-resistivity soil is considered, depending on load conditions and on flashover occurrence. It is also shown that the importance of considering or neglecting frequency-dependent ground parameters on lightning overvoltages transferred to the secondary side of the transformer is minimized by the installation of a medium-voltage surge arrester at its primary side.

This paper presents the design and implementation of the 60-Hz ferroresonant transformer (FRT) for the light emitting diode (LED) street lamp to achieve high-frequency (HF) noise attenuation and ac-line voltage clamping. The HF transformer (HFT) used in the LED driver system for the outdoor LED street lamps cannot effectively suppress the surge current to avoid damage to the LED street lamp. However, instead of the HFT, the low-frequency transformer (LFT) can be applied to protect the LED street lamps from the surge current. The LFT has a higher turns number, which provides larger leakage inductances and parasitic capacitances that effectively suppress and shunt the surge current. With the magnetic shunt, a 60-Hz FRT is proposed to effectively regulate the ac-line voltage to not overstress the LED street lamp systems as compared with the conventional LFT. When the ac-line voltage is higher than the rated voltage, the additional magnetic flux generated from the primary-side winding flows through the magnetic shunt but not the secondary-side winding. The rated output voltage of the secondary side can be sustained to achieve ac-line voltage clamping. A prototype of the LED street lamp with a 60-Hz FRT has been designed and implemented to verify the feasibility of the HF noise attenuation and ac-line voltage clamping.

For isolated dc–dc power converters, the interwinding parasitic capacitance of the transformer is usually one of the main paths for common-mode (CM) noise. In order to simplify the CM noise analysis, this paper proposes a two-capacitor transformer winding capacitance model. The model is derived based on general conditions so it can be applied to different isolated converter topologies. A measurement technique is also proposed to obtain the lumped capacitance for the model. The CM noise models of several isolated converter topologies are analyzed with the proposed two-capacitor transformer winding capacitance model to achieve simplicity. Finally, the proposed transformer winding capacitance model and measurement technique are verified by simulations and experiments.

Isolated high-output current DC/DC converters are critical for future data center power architecture. LLC converters with matrix transformer are suitable for these applications due to its high efficiency and high power density. Different matrix transformer structures are investigated in this paper. To improve the current design practice, a high-frequency transformer loss model is developed and a detailed design methodology is proposed. Moreover, a novel matrix transformer structure is proposed to integrate four elemental transformers into one magnetic core with simple four-layer print circuit board windings implementation and further reduced core loss. By pushing switching frequency up to megahertz with GaN devices, the proposed matrix transformer can achieve high efficiency, high power density, and automatic manufacturing for magnetic components. A 1-MHz 380 V/12 V 800-W LLC converter with GaN devices is demonstrated. The prototype achieves a peak efficiency of 97.6% and a power density of 900 W/in3.

In this paper the control strategy of power electronic transformer (PET) is proposed. The analyzed structure of PET uses two seven-level cascaded H-bridge (CHB) rectifiers. The electrical power of PET is transferred between DC-links of CHB converters using dual-active-bridges (DABs) and low voltage high frequency transformers. The roposed solution allows for controlling the active and reactive power with a low level of harmonic distortions. The DC-link voltages and the load of the utilized H-bridges are controlled using appropriate modulation strategy. The theoretical issues are confirmed by simulation and experimental results.

This paper presents a reduced switch count multipulse rectifier with medium-frequency (MF) transformer isolation. The proposed topology consists of a three-phase push–pull based ac to dc rectifier with a MF ac link employing two active switches. A three-phase, five-limb, multiwinding MF transformer is employed for isolation. The secondary side of the transformer is connected in a zig-zag configuration and is fed to two six-pulse diode rectifiers, achieving 12-pulse rectifier operation. The primary advantage of the proposed system is reduction in size/weight/volume compared to the conventional 60 Hz magnetic transformer isolation rectifier system. Operating the transformer at 600 Hz is shown to result in three times reduction in size. Furthermore, the proposed system employs only two active semiconductor switching devices operating under a simple pulse width modulation scheme. Also, the zig-zag transformer connection helps to balance leakage inductance on the secondary side. Detailed analysis, simulation, and experimental results on a 208Vl-l, 3.15 kW laboratory prototype are presented to validate the performance of the proposed approach.

International Thermonuclear Experimental Reactor (ITER) adopts a pulsed power electric network (PPEN) to feed the pulsed loads including the superconducting coils and the heating and current drive (H&CD) systems, and the PPEN is received from 400 kV grid and transformed to 66 kV and 22 kV distribution voltage levels via 3 main step-down transformers. The short-circuit currents of PPEN transformer are calculated based on theoretical formulas. Then the short-circuit leakage magnetic field is simulated via a two-dimensional finite element analysis software. Finally, the short-circuit electrodynamic forces and temperature rises on windings are calculated and the results are compare with the allowable values in IEC 60076-5. All calculated values of short-circuit electrodynamic forces and temperature rises are within allowable values limits. It is proved that PPEN transformer has enough anti-short circuit strength on windings.

A design methodology of the ohmic heating and plasma current induction system for a small tokamak is presented, capable of inducing in a preionized gas loop an average current of 50 kA over a 20 ms pulse. The design tool consists of a mathematical model that reproduces the transient behavior of a transformer when the primary current is generated by the discharge of a capacitor bank. The model utilizes linked and coupled magnetic fluxes between windings, and includes core saturation effects, as well as the hysteresis effect. A prototype transformer, with known parameters, was used to validate the model. This numerical tool was used in the conceptual design of the ohmic heating system; two combinations of feed circuit and construction parameters were tested, and best results were obtained when the primary solenoid was fed from the discharge of a 540 μF capacitor bank charged to 11.7 kV. The parameters of the solenoid were also determined, taking into consideration size and weight constrains; the best solenoid turned out to be a single-layer, 238-turn, 0.42 m diameter and 1 m long straight solenoid, made with 12 AWG magnet wire. The required steel core has a cross section of 0.1039 m 2 and a weight of 4 tons.

Based on a resonance structure with switching capacitor, a simple power circuit of high frequency controllable electronic supply line transformer having improved mass-size parameters and overvoltage protection of its transistor switches is considered. Analysis of its operation, formulae for calculation of power elements, regulating and load characteristics are presented.

Among the various techniques for transformer protection, more accurate, sensitive, and secure results can be achieved by the flux-based ones. The symmetrical form of the magnetic flux distribution will be disturbed due to any fault occurrence in the transformer windings. So, it seems that this is an appropriate criterion to achieve a suitable protection algorithm, while it can be used to detect faults as well as identify the faulty phase/region. In this paper, a miniature sensor is proposed consisting of simple planar magnetic flux sub-sensors and located in the existing space between the transformer core and the first insulating layer to sense the leakage flux distribution. Comparing the induced voltages in symmetrical sub-sensors for each phase exhibits asymmetrical configuration of the leakage flux distribution and identifies the faulty region in that phase. Appropriate fine and tiny construction of the proposed sensors provides them a good choice to install in the transformer core vicinity. Then, the leakage flux can be sensed in the origin regions during turn-to-turn faults. Experimental results show that such sensors have high sensitivity to detect leakage flux that ejects from the core or comes back to it.

Power transformers are a key component of the electricity supply grid. Monitoring and assessment of their condition has always been of great concern [1]. In critical situations, transformer failures can cause irreversible damage and considerable financial loss to the grid stakeholders or even the end-users [2], [3]. In this regard, mechanical defects are considered a major problem in distribution and power transformers [4]. Consequently various diagnostic techniques have been developed to identify transformer mechanical defects and winding deformation.

This paper presents the analysis and modeling of the non-minimum phase effects that can occur in the low-frequency response of transformers. This effect is highlighted when the phase response has angles outside the range –90° to +90° and the typical response shape, which consists of two resonance points for the lateral phases and one for the central phase, is not found. Analysis based on physical effects, particularly the grounding capacitances and their association with the response, is presented. A model is proposed for characterizing and reproducing transformer low-frequency response, including the non-minimum phase effect. Actual frequency response measurements were obtained experimentally to assess the modeling.

A test stand to evaluate the radial buckling strength of power transformer windings under the influence of electromagnetic forces is presented. The resulting conductor deformation can be measured in parallel to the sinusoidal test current. The proposed test results focus on the inception of forced buckling for continuously transposed conductors (CTCs), which are a special type of conductor often used in power transformer windings. In the literature, there barely exist calculations to the radial buckling withstand capability of CTCs. Therefore, a finite-element analysis-based simulation model for this kind of conductor is proposed and verified by the tests. For this verification, three different CTC types are used. The final simulation model is based on a coupled analysis of magnetostatic force calculation and static structural deformation analysis. It suitably reproduces the measurement results from the dynamic short-circuit tests. Furthermore, a standard formula describing radial buckling phenomena inside power transformers is adapted for use with CTCs.

Results of experimental investigation and design optimization of flyback transformer are presented. Aim of the work is to investigate experimentally the impact of the flyback transformer design on the flux leakage and maximal output power. It is difficult to evaluate the effect of the leakage flux mathematically because it depends on various factors: the position of the windings relative to each other; the position of the windings in the transformer; the distance between the winding. A multi-winding flyback transformer was used for experiments. Using the results of the experiments optimal design of the investigated flyback transformer was defined. Article in Lithuanian. Flyback tipo transformatoriaus sklaidos srauto mažinimo tyrimas Santrauka Darbe pateikti flyback tipo impulsinio transformatoriaus eksperimentiniai tyrimai. Tyrimų tikslas – ištirti, nuo ko priklauso flyback tipo transformatoriaus magnetolaidžio sklaidos srauto (angl. leakage inductance) reiškiniai, kurie mažina tokio tipo transformatoriaus efektyvumą ir didžiausiąją darbinę galią. Matematiškai sunku įvertinti sklaidos magnetinio srauto reiškinį transformatoriaus magnetinėje sistemoje, nes jis priklauso nuo transformatoriaus apvijų tarpusavio išsidėstymo. Todėl eksperimentui atlikti panaudotas daugelio apvijų flyback tipo transformatorius, kuris ištirtas eksperimentiškai. Palyginus gautus eksperimentų rezultatus, nustatytas optimalus tirto flyback transformatoriaus darbo režimas ir optimalūs konstrukciniai parametrai. Reikšminiai žodžiai:flyback transformatorius, magnetinis sklaidos srautas, viršįtampiai, slopinimo grandinė.

As one of the most important components of transmission equipment in electric power field, power transformer bears the important task of energy conversion, power distribution and energy transfer. Because generated a huge force in transformer winding by its interior circuit excitation, it would be taken place easily to distortion, collapse and other damage of its structure, which brings a large safe and reliable problem. The lack of mechanical strength of the coil, which cannot withstand the sudden short-circuit current impact, led to the break of insulating layers, is one of the main causes in transformer failure. In this paper, it is discussed that theoretical and experimental study on mechanical force of transformer coil short circuit. According to the structural mechanics, the calculation of the contacted pressure between the coils is attained by building the contacted model. And the experimental system is been designed, which has a servo control system. Compared the theoretical and the Experimental data, the best accurate computed method for contacted pressure is obtained. It is helpful to improve the resistance ability for short circuit of transformer, and provide guarantee for the safe operation of the transformer.

It would be hard to overstate the importance of transformers in our electrical networks. They’re literally everywhere: on poles and pads, in substations and on private property, on the ground and under it. There are probably dozens in your neighborhood alone. It’s hard to imagine a world without them. But my colleagues and I are doing just that.

Electric arc furnace transformers are very important in the steel production industry. Currently, it is very common for companies to buy used transformers considering the huge investment that represents the acquisition of a new transformer and also the delay in the purchase transaction. Before accepting a commercial agreement and putting the transformer into operation, it is necessary to evaluate the level of wear out due to the electrical and magnetic stresses to which the transformer is subjected. Electrical, mechanical, and insulation tests, that follow procedures stipulated in guides and standards, are performed on used transformers during the purchase process in order to identify incipient faults and to determine their general condition. A methodology is proposed to carry out these tests. In addition, a new model based on fuzzy logic is implemented to assess the level of wear of the transformer, its remaining life, and its feasibility of reuse. The fuzzy input variables of this new model are the results of the tests performed on each element of the transformer.

This paper describes the design, fabrication, and analysis of an ultra-thin and highly flexible current sensing transformer. The transformer windings are printed on flexible polyimide substrate layers with a magnetic foil sandwiched in between. The fabricated transformer can wrap around any electrical cable for current sensing purpose. The design of the transformer adopts a circuit model that includes the nonlinearity of the magnetic core based on the Jiles–Atherton hysteresis model, which is validated experimentally.

An improved algorithm is described for phase comparison in differential protection leg currents to ensure stable operation in the conditions of measuring current transformers saturation. The operation of the algorithm is examined on the example of differential protection of a power transformer with group 11 winding connections.

This paper introduces a three-phase reversible transformer model for the study of low-frequency transients. The model topology is obtained from the direct application of the principle of duality by drawing basic circuit elements on top of the transformer frame. The model parameters are obtained from terminal and physical size measurements as well as the characteristics of the iron core. The implementation of the model is based on standard drag-and-drop circuit elements available in electric circuit simulation programs. The model is validated with inrush current and normal open circuit laboratory measurements. The main advantage of this model is that the parameters remain the same for the simulation of different operating conditions from all terminals. This is true for transients involving deep saturation, normal loading operation, or open circuit.

Purpose The purpose of this paper is to design a current transformer model based on the principle of B-dot. It can reflect the change of transmission line current and meet the requirement of automation and intelligence for current measurement in power system. Design/methodology/approach In this paper, a new type of current transformer is designed on the principle of B-dot, which has the structure of the inverse series of planar air core coils and the form of printed circuit board (PCB). With this structure, the current transformers can induce magnetic field quite well. The finite element simulation for the current transformer with n layers structure is conducted in the Maxwell, which help to optimize the design of the current transformer. Findings By setting up the experimental platform, the experiment of the current transformer is carried out. The results of the test show that the measurement accuracy can satisfy the requirement of measurement. Besides, the new current transformer has good transient characteristics and can meet the needs of the development of smart grid. Originality value The new type of current transformer is based on the principle of B-dot, which is designed with a new type of non-contact PCB hollow coil current transformer. It has no iron core, no ferromagnetic effect and the phenomenon of ferromagnetic resonance. It has great progress in its insulation performance, volume and bandwidth response. In addition, the planar hollow coil of the inverse series structure can make the structure more accurate.

In this paper, an integrated transformer is proposed to provide current sensing in the primary side and passive current phase detection for a self-oscillating contactless resonant converter (SOCRC). The proposed transformer integrates power transfer and current sensing. By using the secondary current phase of a series/series-compensated resonant converter in the control of the inverter, better output controllability, dynamic response, and self-adaptability can be achieved simultaneously. The integrated current sensing winding, being a crucial part of SOCRC, is analyzed in detail. In order to make the output voltage insensitive to the clearance and misalignment conditions, the requirement of the new integrated transformer is given. Experimental results of an 80-W SOCRC prototype verify the analytical results.

The output voltage of capacitor voltage transformer (CVT) is highly influential during initiation of contingencies due to its transient behaviour. This study proposes a novel method for fundamental component phasor estimation which reduces the transient impact of CVT. The proposed method (PM) first, based on hybrid full-cycle averaging window and also least squares (LS) technique, calculates the transient components of the CVT. In the next stage, by subtracting estimated transient behaviour from fault voltage signal, the fundamental phasor component of the obtained signal can be estimated utilising discrete Fourier transformer (DFT). Also, in order to prevent instability due to off-nominal frequency sampling rate, DFT formulation is improved. Several case studies have been carried out in order to show the robustness of the PM in the realistic simulation scenarios. Results show that the PM given in this study is able to estimate fundamental phasor voltage fault signal quickly and precisely with ignorable effect from transient impacts of CVT in almost always one cycle after fault occurrence.

An intentional active power flow control between microgrids in a multi micro grid system in the grid connected mode is achieved at the point of common coupling. The thyristor controlled on load tap changing transformer at the point of common coupling is equipped with voltage based power flow controller where different tapping are switched according to the required power flow condition to be satisfied and the amount of power to be transmitted. Accordingly, the voltage based droop control associated with the distributed generation units in each of the micro grid changes their active power generation. Thus, without much communication to other elements of the micro grids, the Smart Transformer at the point of common coupling allows active power exchange. The simulations performed in MATLAB/ SIMULINK for both three phase and single phase system follows that active power flow control is achieved in a multi micro grid system using the thyristor switched Smart Transformer. Hardware implementation has also been initiated to analyze the concept of power flow control with thyristor switched Smart Transformer and results are studied. The proposed system shows its operational effectiveness, which can be employed to solve grid emergencies in real time.

High frequency planar transformer is a new type of transformer that operates at high frequency. Its significance is proved through its miniaturised features, high power density and other performances. The design of planar transformer needs to consider several fundamental factors, including core material, dimension of layers and winding pattern. As the volume decreases, extra attentions need to be paid in power losses and safety requirements. Different converter topologies, with planar transformer integrated, are also investigated and simulated for maximising the overall efficiency of conversion. Overall, the use of planar transformer in converters can bring several benefits, and, hence, there is a strong interest of comprehending its structure and high frequency behaviours.

This paper presents a new technique for the transient analysis of power transformers under standard impulse excitations. Based on a 2-D finite element method magnetic and electric field simulation coupled to an electric circuit, the technique allows for an analysis of a wide range of winding and insulation designs of a transformer. Also, a 2-D approximation and a direct electric and magnetic field computation (without an equivalent circuit modeling) make this technique effective for engineering applications. Results of calculations of a three-phase power transformer 125MVA-220/10 kV are presented to validate the accuracy of the approach.

This article presents a study of an AC voltage stabilizer based on a three-phase hybrid transformer combined with a matrix converter. The proposed solution is used to control AC voltage amplitude and phase shift. By adjustment of these voltage parameters we can reduce the effects of overvoltage, voltage dips or lamp flicker. Such negative phenomena are very significant, particularly from the perspective of the final consumer and sensitive loads connected to the power network. Often the voltage in the power system can be adjusted using a mechanical or thyristor controlled regulator, which in a stepwise manner switches the taps of the electromagnetic transformer. The method for obtaining continuous control of the voltage magnitude and phase shift with the use of a conventional transformer with two output windings and a matrix converter is presented in this paper. The operating principles, mathematical model and properties of the proposed voltage stabilizers are discussed in this paper. The main part of the article will be devoted to the mathematical model which is based on an averaged equation. Computer simulation results are presented and compared with the results of a mathematical study.